1. Field of the Invention
The present invention relates to a shape measuring machine and a method of correcting a shape measurement error. For example, the present invention relates to a shape measuring machine such as a coordinate measuring machine and a method of correcting an error in its shape measurement.
2. Description of Related Art
These days, shape measuring means such as a coordinate measuring machine (hereinafter, also referred to as CMM) is used to inspect the machining accuracy (or processing accuracy) of a product having a 3D (three-dimensional) shape. Such a CMM carries out shape measurement, for example, by moving a scanning probe along a 3D shape.
For example, when measurement using a scanning probe is carried out by using a shape measuring machine such as a CMM, measurement errors occur due to effects caused by the movement of the slider on which the scanning probe is mounted. For example, when a circle is measured by using a scanning probe, motion errors called “quadrant projections” occur. The quadrant projection is a projection-shaped motion error that occurs when the quadrant of the mechanical orthogonal coordinate system of the measuring machine is changed (when the motion direction on each axis is reversed) in the circular motion of the scanning probe. Further, due to this quadrant projection, a projection-shaped measurement error occurs. The quadrant projection is mainly caused by a backlash and the like resulting from the mechanical structure of the measuring machine.
FIG. 13 shows a measurement result that is obtained by measuring the shape of a ring gage (ring-shaped object to be measured) by using a typical coordinate measuring machine. As shown in FIG. 13, a projection measurement error is observed in the boundary area P1 between the fourth quadrant and the first quadrant and in the boundary area P2 between the second quadrant and the third quadrant in the measurement waveform. They are measurement errors resulting from quadrant projections caused by backlashes and the like that occur when the motion of the scanning probe in the X-axis direction is reversed.
Further, Japanese Unexamined Patent Application Publication No. 2007-315897 discloses an example of a method of correcting an error in measurement using a scanning probe. According to this technique, the position of the tip of the slider is estimated by using a correction filter based on the frequency transfer characteristic between the scale and the slider tip. Then, a measurement value is calculated by adding the estimated value and a probe detection value, so that the measurement error that occurs due to the quadrant projection can be corrected.